1. Field of the Invention
More particularly, the present invention is related to processing unit reclaiming requests in a solid state memory device.
2. Description of Related Art
This application is related to the commonly owned U.S. patent application Ser. No. 13/688,270 entitled “Scheduling Requests in a Solid State Memory Device”, the entire contents of which are incorporated herein by reference. Solid state memory devices encompass rewritable non-volatile memory devices which can use electronic circuitry for storing data. Currently, solid state memory devices replace conventional storage devices such as hard disk drives and optical disk drives in some arenas, such as in mass storage applications for laptops or desktops. Solid state memory devices are also investigated for replacing conventional storage devices in other areas such as in enterprise storage systems. This is because solid state memory devices offer exceptional bandwidth as well as excellent random I/O (input/output) performance and an appreciated robustness due to lack of moveable parts.
However, writing data to a solid-state memory device such as a flash memory device requires paying attention to specifics in the flash technology. NAND flash memory is organized in pages and blocks. Multiple pages form a block. While read and write operations can be applied to pages as the smallest entity of such operation, erase operations can only be applied to entire blocks. While in other storage technologies outdated data can simply be overwritten by up-to-date data, flash technology requires an erase operation before up-to-date data can be written to an erased block.
In flash technology erase operations take much longer than read or write operations a writing technique called “write out of place” is applied in which new or updated data is written to some free page offered by a free page allocator instead of writing it to the same page where the outdated data resides. The page containing the outdated data is marked as invalid page.
The more data that is written over time, the less free pages can be offered and blocks can need to be reclaimed for a free block queue, for example, a queue for providing free and erased blocks for writing new and updated data to. Free blocks need to be reclaimed from blocks filled with valid and/or invalid data. The block reclaiming process—also known in literature as “garbage collection process”—first identifies blocks for cleaning based on a given policy. Then valid data that still resides in these blocks is relocated and copied to other blocks, and finally the blocks that are now free from valid data are erased and become available again for rewriting. Consequently, the block reclaiming process introduces additional read and write operations as well as erase operations, the extent of which depends on the specific policy deployed as well as on system parameters.